High potential terminal



May 3o, 1939.

H. M. HOBART HIGH POTENTIAL TERMINAL Filed Sept lnverwtor:

Hen ry M .Hobart His tto They.

Patented May 30, 1939 UNITED STATES PATENT OFFICE General ElectriCompany, a corporation of Application September 23, 1937, Serial No.165,327

7Claims.

Terminals'or bushings 'as now constructed for high potential -electricalapparatus, such as cables and transformers, are very large and ex'-pensive, especially where large amounts of power are involved. 'Ihls ischiey because of the size of the insulator which is usually made ofporcelain with numerous ribs on the periphery to increase the length ofthe surfacecreepage path. The higher the potential, other things beingequal, the greater is the cost. For some installations, the cost of asingle terminal is several thousand dollars.

'Ihe object of my invention is the provision oi.' an improved terminalorbushing `for high tension apparatus, and specifically one `which issubstantially smailerin size for the same potential than those now inuse and one which is substantially cheaper to construct.

In carrying out my invention, a substantial departure is made from theestablished practice. Instead of insulating the center conductor ot theterminal by wrappings of paper im- Dregnated with oil, the saidconductor is left bare and the insulation thereof eilected by highpressure gas. This pressure will ordinarily'be diil'erent for diierentvoltages but for many installations the gas pressure desirably may be ofthe order of 600 pounds per square inch. The transformer or other tankassociated with the terminal may also contain insulating gas but at amuch lower pressure, say oi the order of 300 .pounds per square inch.The purpose in using a lower pressure in the tank which is relativelylarge, than in the terminal which is relatively small, is to avoidhavingto make the tank unduly expensive in virtue of the thick,wallsrequired. While Iconsider it preferable to employ compressed gas asinsulating media in both terminal and tank, my improved terminal mayadvantageously bel used with apparatus that is submerged mju sued tanks.Another advantage of my improved construction arises from the fact thatthe insulator is small and hence the gas chamberwithin it is likewise ofsmall volume. This permits the use ot gases of the highest insulatingqualities which are the more expensive. The con of such gases for use inlarge tanks would ordinarily be prohibitive on account oi' the-quantityrequired.

In the drawing is illustrated in vertical section a terminal or bushingembodying my invention.

I indicates e. part of a metal tank or casing containing high potentialelectrical apparatus such, for example, as a 'transformer which it isdesired to connect to some outside conductor. From one point of view,the casing can be re- Earded asa metal wall through which itis'desiredto extend a high voltage conductor. The particular apparatus within thetank is not material and for 'that reason hasnotv been illustrated. Thepart o! the casing not occupied by the apparatus is filled withcompressed gas which serves in part to insulate the conductors withinit. rBecause the casing is or may be large, it is desirable to limit thepressure o1' the gas to a-value .which can safely be confined withoutmaking the wallsunduly thick and heavy. The pressure may desirably be ofthe order of 300 pounds per square inch. The upright or neck portion ofthe tank is provided with a. cover 2 which is securely bolted theretowith a sealing gasket between the adjacent surfaces. On the cover issupported the insulator forthe center conductor 3, the lower end ofwhich is con-` nectedby a connector to the electrical apparatus withinthe tank in the ordinary way and the upper end bya connector to anoutside conductor leading to or from other electrical apparatus.- 'I'hechamber t within the insulator is illled with a selected insulating gasunder pressure which is substantially higher than that in the tank,Vysay of the order or 600 pounds'per square inch. Because oi the highpressure, the insulator must be of suilicient strength to withstand saidpressure and is of special construction. This means that the diameter ofthe insulator should be small aswell as highly resistant to internalpressure. Decreasing the diameter introduces the problem ofproperlyinsulating' the center conductor. `The terminal comprises avertical tube 5 made of brous laminated material, as wrapped paper forexample, the layers of which are united by a binder' such as acondensation product, the latter being cured under heat and pressure toform a rmstrong body. The lower end oi' the tube is internally screwthreaded to receive a cap'i which may be of the same material as thetube, said cap having a recessl through which the conductor I extends.In the cap is located a well packed screw threaded plug 8 that forms aseal for the lower end of the conductor 3. It is desirable that thematerial of the vertical tube shall have a high dielectric constantbecause the stress in a material is inversely as its dielectricconstant, and it is desired to` keep it low in the solid material andhigh in the gas.

At a suitablev portion of its length, the tube is somewhat enlarged toform two shoulders 9.

Surrounding the enlarged portion is a metal sleeve I which is sealed toor forms a part of the cover 2. The sleeve is externally screw threadedat its ends to receive gland nuts II and I2, said nuts having internaliianges which flrmly-'f'press packings I3 against the ends of the sleeveto form pressure tight joints. `Surrounding the tube above the nut I2 isa -body of insulation It of any suitable character. The upL per end ofthe tube is provided with a series of ring shaped barriers I5 made ofinsulating material and desirably of the same kind as the tube itself.Between the barriers are small spacing rings I6. The barriers materiallyincrease the resistance to passage of current from the exposed end ofthe center conductor to the metallic tank or casing, and also serve toreinforce' the tube.

'Ihe upper end of the tube 5 is internally threaded to receive thethreaded end of an insulator I1 which may be made of strong glass,

porcelain or equivalent insulating material. The

insulator iskprovided with parallel ribs I8 which not only increase thelength of the current creepage surface thereof but also strengthen it aswell. Because of' the small surface of the wall of the bore of theinsulator, it is capable of withstanding high internal pressure.

Where the center conductor passes out through the insulator, a gland nutI9 is provided and also a compressible packing 20. 'I'hese parts may betaken as illustrative of the parts at the lower end of center conductor.Since the upper end projects out into the surrounding atmosphere whichmay be humid or dirty or both, the distance between the conductoremerging at I9 and` the cover 2 of the metal tank must be great for highvoltages in order to avoid surface leakage.

' The distance between the lower gland nut I I and 5, cap 6 andinsulator I1 may be regarded as a the bottom plug 8 must be proportionedso as not to exceed the volts per mil of disruptive strength of thematerial of the tube 5. Gas is admitted to the small chamber 4 withinthe terminal by a supply pipe 2I subject to the control of a valve 22,said pipe being threaded into the metal sleeve I0.

In the broader aspect of my invention, the tube completely closed vesselirrespective of the material of which it is composed, having the centerconductor permanently sealed at the'points where it passes'through theends of the vessel. Also that with effective seals to prevent leakage,the vessel may be charged once and for all. Under this condition, theinlet pipe 2i may not be required.

Various gas and mixtures thereof having high dielectric strength areavailable for insulating the centerconductor I within the chamber oftheterminal.' For example, I may use carbon tetrachloride gas, CC14, eitherby itself or mixed with air or other gas. With a gas such as CCM atatmospheric pressure,'i'f maintained above itsv liquefying temperatureof I7 deg.C. a gap or space between bodies. of 1.5 millimeters is equalto a gap of 30 millimeters between such bodies underl atmospheric airconditions. As the pressure of the gas is increased and ifl itstemperature is also-in-A creased by the amount necessary to prevent itsliuuefying at the increased pressure. its value as a dielectric verygreatly increases. Several gases which are especially suitable furthepresent purposes tend when cold to become liquid under the highpressures employed. Nitrogen gas under suitsblepressure may also be usedas a filling for the chamber of the terminal but will not provide suchgreat dielectric strength as CC14 gas. I do not by the foregoing intendto be understood as exhausting the subject of suitable gases but merelyrefer to them as examples. Also it is to be understood that a mixture oigas may be employed, provided they have a sufliciently high dielectricstrength for the voltage to which the center conductor is subjected.

Certain of the gases 'which are available for the present purpose andwhich are characterized by a particularly high dielectric strength havea relatively high liquefaction temperature, that of CCli being 77 C. atatmospheric pressure. At 400 pounds absolute, it is 190 C. In order tomaintain the insulating material in s. gaseous state while underrelatively high pressure, a heating means is employed which may takevarious forms. In some cases, some or all of the necessary heat may beprovided by the heat generated by the electric current in its passagethrough the center conductor, or the heat generated may in whole or inpart be due to external means. For example, the center conductor mayhave its resistance suitably increased or it may be composed wholly orin part of magnetic material. Surrounding the terminal at a. suitablyselected point and supported in any desired manner is a coil ofinsulated wire 23 which is energized by a low potential source ofalternating current, such as is available in transformer containingpower stations. The current passing through the coil acting by inductioncauses lines oi force to pass through the iron of the center conductorand induce eddy currents and hysteresis therein which result in raisingits'temperature, and this elevated temperature may be made sufficient tomaintain the insulating fluid within the chamber 4 in a gaseouscondition. The effect can be intensifed by using in coil 23 a current ofhigh periodicity instead of the usually available 60 'cycle current. Butforv moderate heating, the convenient and always available 60 cyclecurrent will suillce.

According to present practice, the center conductor of terminals is madeof low resistance material so asLto reduce losses, but for the purposesof the present invention, I depart from said practice to the extentnecessary to heat the gas contained in the conductor enclosing vessel.Where a considerable amount of heat is required to prevent liquefactionof the gas under the determined pressure, I may make the whole or aportion of the conductor of high resistance material such as thatcommonly referred to as nichrome. In some cases, the vessel may containa surplus of liquid C014 and the amount which evaporates will bedependent upon the temperature and pressure.` An increase ofpressure-increases the dielectric strength of the gas. Aspreviouslyjndicated, the heat generated by the center conductor may formthe sole heating means for the gas, or the heat for the gas may begenerated by means external to the vessel or both internal and externalmeans may be utilized for the purpose.

While certain materials and gases Ihave been indicated or suggested inthe foregoing as suitable for the purposes hereof, it is to beunderstood that in their selection care should be exer- 1d conductorbare, it follows that its diameter can in the chamber ofthe insulatori'or insulatingy the conductor that at the operating temperature- 'thediameter of the conductor permits the radial depth of the gas spacewhich in turn permits decreasing the overall diameter oi the .vOlts andupwaidf It 'will be noted that there is no insulation, such as the usualpaper wrapping, applied to the center conductor. By leaving the be smalland hence that of the enclosing insulator. In referring to the fact thatthe center conductor is uninsulated, I mean that it has no externalcovering which of itself ailords the necessary insulation or a coveringwhich contributes to any substantial extent in insulating the conductor.The tube l constructed as described great strength and one well adaptedto withstand the heavy pressureto which it is subjected. By arrangingthe lower end oi' the porcelain or glass insulator I'l in telescopicrelation with the tube, said insulatoriswell reinforced at thepoint ofconnection. By decreasing the diameter of the upper part oi thechamberor bore within the insulator I'Isoastoreducetheexposedarea,theeii'ectivepressuretendingtobursttheinsulator is reduced to a safe value.

What Iciaim as new and desire to secure by Letters Patent o! the UnitedBtatesis:

1. Aterminal i'or high-potential electrical apparatus an uninsulatedconductor op- Ierating at approximately 100,000 volts and upwards, asealed vessel through which the conductor extends and projects therefromat both ends, said conductor being sealed at the points where it'entersand emergesi'rom the vessel, a nllingofinsulatinggasiortheveeimaintainedundersuchhighpressurethatittendstobecome liquid when cold, and meanssupplying suiiicient heattothegaswhen current isilowingthrough theconductor to gasiiy said liquid and maintain it in a gaseous state.

2. A terminal for high potential electrical apparatus comprising anuninsulated conductor. a chambcred insulator enclosing the conductor andclosedatbothofitsendasealingmeansforthepartsoi'theconductorpamingthroughsaidciosed ends,aiillingotgaslmdersuchhilhmottheconductoritisliquimandmeansfcrimpartingsumcientheattothebodyoi'gastomaintainitina-gaseousstatelmderoperating conditions whilecurrentisiiowingthroughthe conductor a sealed vessel through which theconductor extends and projects at both ends, said conductor being sealedat the points where it emerges from the vessel, and a iilling ofinsulating gas of low thermal conductivity under high pressure for thevessel which tends to become liquid under such high pressure when cold,the heat from the conductor when current is ilowing through it beingsuiilcient to vaporize said liquid.

4. A terminal for high potential electrical apparatus comprising anuninsulated conductor, a. sealed vessel through which the conductorextends and projects at both ends. said conductor being sealed at thepoints where it emerges from the vessel, a iilling of insulating gas forthe vessel under high pressure which tends to liqueiy when Jccld, andmeans external to the vessel for heating and vaporizing the liquidwithin the vessel when the terminal is in service.

5. A terminal for high potential electrical apparatus comprising anuninsulated conductor, a sealed vessel through which the conductorextends and projects at both ends, said conductor being sealed'at thepoints where it emerges from the vessel, a filling of insulating gas forthe vessel under high pressure which tends to liqueiy when cold, andmeans both inside and outside oi .the vessel for heating and vaporizingliquid within the vessel when the terminal is in service.

6. A terminal for high potential electrical apparatus comprising anuninsulated conductor of magnetic material, a chambered insulatorenclosing the conductor and closed at both ends, said conductorextending through the insulator at said ends, sealing means for theparts of the conductor passing through said closed ends, a illling ciinsulating gas under high pressure in the chamber oi the insulator, saidgas tending to yliquefy under the pressure to which it ris subA jected,and a coil oi wire surrounding the insulator through which current incauses heating by induction of the central conductor to preventliqueiaction of the gas under operating conditions.

'1. A terminal for high DOtential electric conductors comprising aninsulator having a central chamber, one end ofthe insulator beingexposed to a cooling atmosphere, a conductor in the chamberextendingaxially o! the insulator, seallng means for the conductor whereit extends throughthewallsoitheinsulatonaiiliingoi high pressureLinsulating gas tor the chamber which tends to liquefy when itstemperature is reducedbysaidatmosphere,andaheatingmeans iormaintainingthe gas above its liqueiying temperature during normal operatingconditions oi HENRY Il. H OBART.

